Effects of fin arrangement on the melting process in a vertical ‎double-tube heat exchanger considering intermittent ‎boundary conditions

This paper presents a numerical analysis of the solid-liquid phase change within a vertical double-tube heat exchanger containing a phase change material, considering intermittent boundary conditions with the application of the enthalpy-porosity technique. To enhance the rate of heat transfer, copper fins are integrated into the inner wall of the heat exchanger in both uniform and non-uniform arrangements. While the uniform placement of fins at equal intervals accelerates the melting process, it leads to a portion of the phase change material remaining solid at the bottom of the heat exchanger due to weakened natural convection. Conversely, positioning a greater number of fins with a non-uniform distribution at the bottom of the heat exchanger expedites the overall melting process. It is observed that compared to a finless heat exchanger and under constant boundary temperature, the complete melting time is reduced by 53%, 69%, and 75% for uniform fin distribution, and fin distribution with geometric progression q=2 and q=3, respectively. Furthermore, the findings showed that natural convection leads to a greater increase in liquid fraction during melting compared to the assumption of pure conduction. Specifically, liquid fraction increases by about 40% with natural convection and around 15% with pure conduction during the first melting period. While the decrease in liquid fraction is almost equivalent for both conditions during freezing.